Matt Cutts wrote an article with a couple of things you probably didn't know about Google:
Google makes lots of improvements that most people never notice.
Some people think that Google changes a few things every few months. At least in search quality, it’s more like a few things every week. From the article: “the search-quality team makes about a half-dozen major and minor changes a week to the vast nest of mathematical formulas that power the search engine.” I don’t think we’ve discussed our pace of search quality changes before.